Computer Science A level notes, prepared by Dr. Hamdeni
13.1a: Show understanding of why user-defined types are necessary
What is a user-defined data type?
A user-defined data type is created by the programmer so the program’s data matches its requirements exactly.
After defining the type, the programmer can declare variables of that type and use it.
Using user-defined data type to restrict allowed day values Imagine we need a day value that accepts only real days. The seven days can be defined as a type listing Monday to Sunday. A variable today uses this type, and setting today = Wednesday is valid. Result: values outside the list are rejected.
User-defined types fall into two families. A non-composite type is defined without referring to any other type. A composite type is defined with reference to at least one other type. Details about these two families will be in next sections.
Why user-defined types are necessary
User-defined types are necessary when built-in types are not sufficient to express the program’s specific data or constraints.
Defining a type lets the programmer state exactly which values and structure are allowed.
User-defined types add flexibility and help organize complex data, which can improve code readability and maintainability.
Built-in types are standard compartments. A user-defined type is a suitcase you redesign to fit your items exactly.
13.1b: Define and use non-composite types including enumerated, pointer
A non-composite user-defined type is defined without reference to any other type. Two examples of non-composite user-defined types in the syllabus are the enumerated type and the pointer type.
Pointer types
Pointer type stores the memory location of a value. To read through a pointer, go to that location and fetch the value.
The pointer’s definition states the type it points to. You can declare a pointer, store a location in it, and read the value via the pointer.
Pointers let programs refer to values indirectly, which is useful when building data structures that can grow or shrink while the program runs.
Pointer pseudocode TYPE TIntPointer = ^INTEGER DECLARE MyPointer : TIntPointer
Enumerated types
Enumerated type is a type where all possible values are explicitly listed by the programmer with an implied order. You can later declare variables of this type.
Because there is an order, the program can compare listed values, such as checking whether one value comes after another in the sequence.
Enumeration pseudocode To represent days of the week using an enumerated type: TYPE TDays = (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday) DECLARE Today: TDays Today = Wednesday
Seasons enumeration pseudocode TYPE Season = (Spring, Summer, Autumn, Winter)
Pointer usage with an enumeration TYPE Season = (Spring, Summer, Autumn, Winter) DECLARE ThisSeason : Season DECLARE NextSeason : Season DECLARE MyPointer : TIntPointer ThisSeason ← Spring MyPointer ← ^ThisSeason NextSeason ← MyPointer^ + 1 Result: NextSeason becomes the value immediately after ThisSeason in the enumeration order.
An enumeration is like a short menu. You must choose one listed dish. A pointer is like the table number that tells you where the dish is, not the dish itself.
13.1c: Define and use composite data types including set, record and class/object
A composite user-defined type is defined with reference to at least one other type. Composite user-defined types include sets, records, and classes (objects).
Records
A record groups named fields that match the program’s data.
Each field in a record can be of a different data type.
Record for a flight We want to design a flight as a record data type. We will declare the following fields: FlightNumber (integer), FlightTime (minutes), StartLocation (text), EndLocation (text), Plane (code). One example instance Flight1 has: • FlightNumber = 7837 • StartLocation = “Berlin” • EndLocation = “London” • FlightTime = 110 • Plane = “D242”.
Record pseudocode TYPE StudentRecord DECLARE LastName : STRING DECLARE FirstName : STRING DECLARE DateOfBirth : DATE DECLARE YearGroup : INTEGER DECLARE FormGroup : CHAR ENDTYPE
Classes
A class groups properties (attributes) and methods (procedures or functions) that its objects can use.
Simple class design (Pet) Suppose we design a class Pet with properties PetName, PetType, OwnerTel (text), UniqueID (integer), regDate (date). Methods include CreatePet, which assigns a unique ID and sets regDate, and GetContact, which returns PetName and OwnerTel. This shows how a class combines data and behaviour.
Sets
Set is an unordered collection. Use set operations to combine or compare sets.
Union combines two sets into one set that contains every element that appears in either set (no duplicates).
Intersection keeps only the elements that are present in both sets.
Each element in a set is unique; duplicates are not stored.
Set pseudocode TYPE LetterSet = SET OF CHAR DEFINE Vowels ('A','E','I','O','U') : LetterSet
13.1d: Choose and design an appropriate user-defined data type for a given problem
An appropriate user-defined type matches the scenario’s data:
Fixed, known list of values → choose an enumerated type.
Several labelled pieces of data together → choose a record.
Many similar items with actions → choose a class.
Need unique membership checks and set operations → choose a set.
Need to store a location (address) of a value → choose a pointer type.
Matching scenarios to types A fixed set of colours to represent in a program: an enumeration suits this because it lists every allowed value. Details about each house for sale: a record with one field per detail to keep related data together. Storing locations of integer values in memory: a pointer type to hold the location of those values.
If you can list every valid value, choose an enumeration. If you need several labelled pieces together, choose a record. If you will create many similar items with actions, choose a class. If you need unique membership checks, choose a set. If you need to store locations of values, choose a pointer.
